The present Invention relates to a transmission system for providing power to equipment housed in a Faraday cage, to a power source for such equipment, and to a Faraday cage provided with such a power source. Particularly, but not exclusively, the transmission system enables power to be provided to communications equipment housed in a Faraday cage which requires shielding against electromagnetic interference and leakage up to at least 5 GHz.
Equipment which is sensitive to electromagnetic interference is often protected by isolating apparatus within a conductive shell, or Faraday cage. Faraday cages are also used to retain electromagnetic radiation which is emitted by equipment. Electromagnetic field theory describes how electromagnetic waves interact with a conductive surface shell. Essentially, the shell distributes the electrical field around the shell surface, but not through the shell surface. Accordingly, electromagnetic fields generated outside a Faraday cage cannot penetrate within the cage, and internally generated fields cannot escape outside a Faraday cage.
Any nonconductive medium in the surface of the conductive shell provides an effective xe2x80x9cholexe2x80x9d through which electromagnetic waves can escape, FIGS. 1A and 1B of the accompanying drawings Illustrate this effect at a basic level. In FIG. 1A, a positive point charge 10a is shown outside a conductive xe2x80x9cboxxe2x80x9d, Faraday cage 12. Field lines 14a are not able to intrude substantially through gap 16 in the Faraday cage 12. Similarly, as FIG. 1B shows, field line 14b from an internal point charge 10b cannot extrude substantially into the exterior of the Faraday cage 12 through gap 14.
Reducing the size of the gap 16, reduces the amount of external/internal field which is able to gain access to the interior/exterior of the Faraday cage 12. Accordingly, in practice, providing a gap in a Faraday cage is less than a wavelength across, the amount of electromagnetic radiation which is able to compromise the shielding provided by the cage is small.
Faraday cages can thus have a mesh-type structure, or be perforated for cabling access etc., and still function effectively as a shield against most electromagnetic radiation. The Faraday cage will not providing any shielding, however against radiation which has a substantially shorter wavelength than any gaps or other irregularities in the cage surface structure. This provides a limit on the size of any noninductive feature on the Faraday cage surface, whether an air-gap or another type of irregularity.
Commercially, it is advantageous if communications equipment can be isolated from electrical or electromagnetic interference (EMI). EMI can be defined as undesired conducted or radiated electrical disturbances, such as other electronic equipment might generate including transients, which can interfere with the operation of electrical or electronic equipment such as electrical power supplies. EMI disturbances can occur anywhere in the electromagnetic spectrum. However, radio frequency interference (RFI), usually defined as between 24 kilohertz (kHz) and 240 Gigahertz (GHz), is often considered to be synonymous with EMI for practical purposes. For communications purposes, however, providing protection against electromagnetic pulses up to 5 GHz is usually considered sufficient.
EMI/RFI can be generated by a range of phenomena, and include potentially destructive electromagnetic phenomena, both natural and man-made. Lightning and other types of electromagnetic pulses can create very destructive EMI as the pulse fields couple to electrical radiation and generate electrical transients in power and communications systems. For example, some electromagnetic fields can generate very short high energy transients in electrical connections and cables or longer transients which have lower voltage disturbances. For example, core current transients up to around 150 Amp having full width half maximum FWHMs of 1.2 to 1.4 s, and core voltage transients of up to around 7 kV (FWHM 2.1 s to 4 s) can be induced which are known to cause damage. Similarly, transients which have lower core currents ( less than 40 Amp) can be induced which can last for much longer (FWHM 38 to 40 s), and can accompany voltage pulses as high as 9.4 kV, which last for over a minute and so are potentially very destructive. Such transients can generate energy dumps sufficiently large to melt electrical connections and cables.
It is particularly highly desirable to shield communications equipment from such EMI/RFI due to the fact that communications equipment, even if predominantly provided by optical components, often includes electronic components such as integrated circuit (IC) chips, IC packages, and multi-chip modules along with hybrid components.
One solution to the problem of providing protection against such EMI is to filter out the disturbance at the equipment level, as shown in U.S. Pat. No. 6,034,855. However such filters can be relatively costly and expensive to maintain. Another solution is to house equipment in Faraday cage structures which act as shields against EMI.
Faraday cages can have a variety of forms and can be constructed on a variety of scales. At the circuit board level, a conductive shield can be provided by a xe2x80x9ccanxe2x80x9d covering components and electrically grounded to a substrate such as a printed wiring board. At a larger scale, entire racks of equipment may be housed in conductive cabinets which are grounded to shield their contents from external sources of EMI. Other examples of the forms of Faraday cages include conductive housings, temporary large scale structures effectively providing a xe2x80x9cconductive tentxe2x80x9d, a metallic or metalised box or case. The cage surface may be smooth or may be perforated in such a way as to minimise the radiation which is able to penetrate the interior of the cage at any given frequency.
In the field of fibreoptic communications, transmission equipment is often shielded by installing the equipment in a conductive housing. The conductive housing functions effectively as a Faraday cage. Cabling conduits for the housing must be sized appropriately according to the desired cut-off frequency for the Faraday cage, i.e., the conductive surface must not be interrupted for more than a fraction of the maximum wavelength the housed equipment can tolerate. For the purposes of most communications equipment, a hole of around one tenth of the cut-off wavelength is possible without unduly compromising the efficiency of the cage.
However, a problem exists in that power must still be provided to the interior of the cage. Batteries and generators may be provided within some Faraday cages as isolated sources of power, however, these are costly to run and maintain and can take up the space within the Faraday cage. However, in the case where a cage needs to be completely sealed, an isolated power source is required.
In the case where a Faraday cage can be perforated, a direct connection with an external power supply is usually not suitable unless a low-pass filter is provided. Such a filter needs to be able to remove any undesirable power characteristics in the external supply, and has to be able to cope with a range of EMI disturbances. It is essential that such filters are capable of removing electromagnetic power surges, pulses, or spikes, whether created by human endeavour or by natural means such as lightening, electrical storms, etc., within the design tolerance required for the Faraday caged equipment. Accordingly, such filters are often a costly solution to the problem of providing power to equipment housed in a Faraday cage. Other problems include the space required by such filters and the inherent uncertainty in their performance providing protection against higher energy transients.
The invention seeks to provide a transmission system for power to equipment housed within a Faraday cage. The system transfers power from an external power source to an internal power generator and yet does not mitigate the shielding provided by the Faraday cage against EMI disturbances.
A first aspect of the invention seeks to provide a transmission system for providing power to equipment housed within a Faraday cage, the transmission system comprising: a transmission element adapted to transfer energy from a first power source located outside the Faraday cage to a second power source located within the Faraday cage, the transmission element including a conductive region arranged to form conductive contact with said Faraday cage; the second power source for providing said equipment with power, wherein energy is transferred into the Faraday cage along a non-conductive path
Preferably, said transmission element includes a conductive region for forming conductive contact with said Faraday cage.
Advantageously, by providing conductive contact between said transmission element and said Faraday cage, the shielding providing by the Faraday cage is not unduly compromised. A sufficiently good contact, for example, enables the Faraday cage to provide a very good level of shielding.
Preferably, said conductive contact is formed by at least one friction-reducing element provided between the transmission element and a boundary of the Faraday, said at least one friction-reducing element taken from the group consisting of a conductive lubricant, a conductive ball-bearing, graphite, and mercury.
Preferably, said transmission element transfers energy in a form taken from the group consisting of: mechanical energy, rotational energy, hydraulic energy, magnetic energy.
Preferably, said transmission element drives said second power source using said first power source, and said transmission element is taken from the group consisting of: a drive belt; a drive shaft; a fluid; a hydraulic piston; a resilient member; a spring.
Preferably, said transmission element passes through an aperture in said Faraday cage.
Preferably, said transmission system and said Faraday wall are configured to engage each other.
A second aspect of the invention seeks to provide a power source for equipment housed in a Faraday cage, the power source including: a first power source located outside the Faraday cage; a second power source located within the Faraday cage for providing power to said equipment; and a transmission system comprising a transmission element adapted to transfer energy from a first power source located outside the Faraday cage to a second power source located within the Faraday cage, the transmission element including a conductive region arranged to form conductive contact with said Faraday cage; the second power source being providing said equipment with power, wherein energy is transferred into the Faraday cage along a non-conductive path.
Preferably, the power source further includes a low-pass filter.
Preferably, said low-pass filter filters out power disturbances provided to said first power source. Advantageously, this prevents the first power source from being affected by power surges or suffering damage due to high energy EMI pulses coupling to the power source of said first power source.
Preferably, said second power source provides electrical power.
Preferably, said second power source comprises an electrical generator which is driven by the energy transferred by said transmission system.
The first power source may be powered by electrical power. The first power source may be an electric motor.
A third aspect of the invention seeks to provide a Faraday cage for housing equipment and shielding electromagnetic radiation, the Faraday cage having a power source including:
a first power source located outside the Faraday cage;
a second power source located within the Faraday cage for providing power to said equipment; and
a transmission system comprising a transmission element adapted to transfer energy from a first power source located outside the Faraday cage to a second power source located within the Faraday cage, the transmission element including a conductive region arranged to form conductive contact with said Faraday cage; the second power source being for providing said equipment with power, wherein energy is transferred into the Faraday cage along a non-conductive path.
Preferably, said power source provides electrical power to said equipment, said Faraday cage provides optical means for the ingress to and egress from data from said equipment and said equipment is take from the group including: communications equipment, computer equipment, medical equipment.
A waveguide may be attached to said Faraday cage in the vicinity of an aperture through which said transmission element passes, said wave-guide being adapted to prevent further propagation of electromagnetic radiation of a predetermined wavelength away from said aperture in said Faraday cage.
Preferably, said Faraday cage provides shielding against EMI disturbance.
The fourth aspect of the invention seeks to provide a communications network including at least one Faraday cage for housing communications equipment and shielding electromagnetic radiation, the Faraday cage provided with a power source located within the Faraday cage for providing power to said communications equipment; and a transmission system comprising a transmission element adapted to transfer energy from a first power source located outside the Faraday cage to a second power source located within the Faraday cage, the transmission element including a conductive region arranged to form conductive contact with said Faraday cage; the second power source being for providing said communications equipment with power, wherein energy is transferred into the Faraday cage along a non-conductive path.
The Faraday cage is able to provide shielding against electromagnetic radiation over a range of frequencies at which EMI disturbance can affect communications equipment, for example, electromagnetic radiation up to at least 800 MHz, but more preferably up to at least 5 GHz, and even as high as 240 GHz.
Advantageously, such a Faraday cage prevents electromagnetic transmissions emanating from within the cage from being detected externally which, for example, might provide information on the content of such transmissions and the location of the equipment.
Advantageously, the isolation of the power supply therefore enables Faraday cages to be constructed which are particularly suited to environments where confidential information is being transmitted. It is possible to elicit information from electromagnetic emissions, and by enabling the construction of Faraday cages which provide a high shielding level against electromagnetic radiation associated with the transmission of information, protection against detection can be provided for such confidential information.
Advantageously, the power supply can be provided to equipment which is relatively compact in dimensions by suitable modification in size and or the driving mechanisms for the drive shaft. Accordingly, the power supply can be made portable or manually operated. The power supply is thus suitable for deployment in a moveable (e.g. a portable Faraday cage) or a static environment.
The invention is also directed to a method of transferring power from an external power source to an internal power source by which the described apparatus operates and including method steps for carrying out every function of the apparatus.
The preferred features may be combined as appropriate, as would be apparent to a skilled person, and may be combined with any of the aspects of the invention.